Photovoltaic Devices and Methods

ABSTRACT

Photovoltaic devices, and methods of fabricating photovoltaic devices. The photovoltaic devices may include a first electrode, at least one quantum dot layer, at least one semiconductor layer, and a second electrode. The first electrode may include a layer including Cr and one or more silver contacts.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.18/059,803, filed Nov. 29, 2022, which is a divisional of U.S. patentapplication Ser. No. 16/659,626, filed Oct. 22, 2019, now U.S. Pat. No.11,522,094, which claims priority to U.S. Provisional Patent ApplicationNo. 62/749,783, filed Oct. 24, 2018. The content of these applicationsis incorporated by reference herein.

BACKGROUND

Polymer solar cells (PSCs) have attracted attention due to their ease ofprocessing, low cost, flexibility, and/or lightweight nature compared tomany traditional inorganic solar cells. However, the thicknesses of thematerials used in polymer solar cells typically is limited due to theirhigh absorption coefficient. For this reason and/or others, theefficiency of organic solar cells (OSCs) is still very low compared tothe silicon solar cell.

There have been various methods implemented, such as annealing, devicestructure tuning, and active material modification, to improve theefficiency of PSCs. Among the various methods involving two or moreorganic junctions, PSCs with tandem structures have been attempted.Furthermore, photovoltaic devices that include a mixture of inorganicnanoparticles and conjugated polymers, which are referred to as hybridsolar cells, have the ability to absorb near-infrared light.

Adjusting the thicknesses of active layers in tandem photovoltaic cellsis one way to alter their performance. The optimization of a tandemstructure using trial and error experiments, however, is usually costlyand/or ineffective. Simulation, therefore, may be a more effective toolto create improved tandem device structures. OSC devices generallyinclude an organic layer arranged between two different metalelectrodes. A bulk heterojunction (BHJ) organic solar cell typicallyincludes three components: an active layer, a band alignment layer, andelectrodes. The active layer typically is a homogeneous mixture of donorand acceptor materials. The donor materials are generally conjugatedpolymers, whereas the acceptor materials are typically fullerenederivatives. The power conversion efficiency of a promising structure(i.e., a P3HT:PCBM bulk heterojunction solar cell) has been reported tobe 5% (Wang, K.; Liu, C.; Meng, T.; Yi, C.; Gong, X. Inverted OrganicPhotovoltaic Cells, Chem. Soc. Rev. 2016, 45, 2937-2975). It has beendemonstrated that hybrid solar cell can absorb light having a wavelengthup to 800 nm. Another study has showed that a one-junction polymer solarcell with a P3HT:PCBM active layer can cover the 800 nm light spectrumwith 2.9% efficiency. Yet another study has showed that a one-junctionpolymer solar cell having a MEHPPV:PCBM active layer can cover the 800nm light wavelength, and only produces a current density of 6.82 mA/cm².In most, if not all, reports to date, the simulation and optimizationfor these devices were conducted for one-junction PSC cells.

There remains a need for improved methods for simulating and/oroptimizing devices, such as photovoltaic cells.

The semiconductor quantum dots (QDs) of groups IV and VI include thecompounds PbSe and PbS. These semiconductors are commonly known to actas photo-absorbers in at least the near-infrared and visible regions ofthe light spectrum. PbS QDs have achieved recognition for the generationof multiple excitons, huge bandgap tunability, and/or relatively easysolution methods.

Colloidal QDs (CQDs) are generally inorganic semiconductor materialswith organic molecules on their surface. By using a surface treatment,these materials may perform in a manner that is similar to either apositive (p-type) or negative (n-type) semiconductor. This feature maypermit their usage in the architecture of optoelectronic organic,inorganic, and/or hybrid devices. PbS QD materials have been used andstudied recently in applications in bilayer photodetectors (see, e.g.,Ren, Z.; Sun, J.; Li, H.; Mao, P.; Wei, Y.; Zhong, X.; Hu, J.; Yang, S.;Wang, J. Bilayer PbS Quantum Dots for High-Performance Photodetectors.Adv. Mater. 2017, 29, 1702055), solar cells (see, e.g., Wang, R. L.; Wu,X.; Xu, K.; Zhou, W.; Shang, Y.; Tang, H.; Chen, H.; Ning, Z. Highlyefficient inverted structural quantum dot solar cells, Adv. Mater. 2018,30, 1704882; Ganesan, A. A.; Houtepen, A. J.; Crisp, R. W. Quantum DotSolar Cells: Small Beginnings Have Large Impacts, Appl. Sci. 2018, 8,1867; and Bi, Y.; Pradhan, S.; Gupta, S.; Akgul, M. Z.; Stavrinadis, A.;Konstantatos, G. Infrared Solution-Processed Quantum Dot Solar CellsReaching External Quantum Efficiency of 80% at 1.35 μm and Jsc in Excessof 34 mA cm-2, Adv. Mater. 2018, 30, 1704928), cell imaging, andlight-emitting diodes. The photovoltaic device architecture and the QDsurface ligands may, in some instances, play a role in determining theoptoelectronic properties of QD solar cells (see, e.g., Kim, B.-S.;Hong, J.; Hou, B.; Cho, Y.; Sohn, J. I.; Cha, S.; Kim, J. M.Inorganic-ligand exchanging time effect in PbS quantum dot solar cell,Appl. Phys. Lett. 2016, 109, 063901; Tang, J.; Kemp, K. W.; Hoogland,S.; Jeong, K. S.; Liu, H.; Levina, L.; Furukawa, M.; Wang, X.; Debnath,R.; Cha, D.; et al. Colloidal-quantum-dot photovoltaics usingatomic-ligand passivation, Nat. Mater. 2011, 10, 765). CQD materialsconsist of individual QDs where the QDs remain side by side (see, e.g.,Balazs, D. M.; Dirin, D. N.; Fang, H.-H.; Protesescu, L.; ten Brink, G.H.; Kooi, B. J.; Kovalenko, M. V.; Loi, M. A., Counterion-MediatedLigand Exchange for PbS Colloidal Quantum Dot Superlattices, ACS Nano2015, 9, 11951-11959; and Szendrei, K.; Gomulya, W.; Yarema, M.; Heiss,W.; Loi, M. A. PbS nanocrystal solar cells with high efficiency and fillfactor. Appl. Phys. Lett. 2010, 97, 203501).

The surface morphology may, in some instances, play a role in achievingan efficient PbS QD solar cell, however, the surface-area-to-volumeratio becomes higher in QD materials, which can cause electronic traps,which, in turn, can increase the likelihood of charge recombination(see, e.g., Lan, X.; Voznyy, O.; Garcia de Arquer, F. P.; Liu, M.; Xu,J.; Proppe, A.; Walters, G.; Fan, F.; Tan, H.; Liu, M.; et al. 10.6%Certified Colloidal Quantum Dot Solar Cells via SolventPolarity-Engineered Halide Passivation, Nano Lett. 2016, 16, 4630-4634).

To achieve continuous charge transfer and separation, the solar cellsurface morphology may be considered. The film quality can depend on anumber of factors, including, but not limited to, the quantum dot size,the concentration of QD material, the ligand exchanger and/or exchangingtime, an annealing process, band alignment, solvent properties, or acombination thereof. The rate of evaporation, viscosity, and/ordispersibility may be properties of the solvent that may be adjusted inorder to obtain pinhole-free and/or crack-free surfaces. Changing theligand exchange time can also contribute to surface quality. Properbandgap alignment of the device material can prevent chargerecombination, thereby possibly reducing series resistance.

Extensive research on solar cells, including PbS QD solar cells, hasbeen conducted, and most, if not all, of the research, has focused onsolvent engineering and bandgap alignment (see, e.g., Chuang, C.-H. M.;Brown, P. R.; Bulovic', V.; Bawendi, M. G. Improved performance andstability in quantum dot solar cells through band alignment engineering,Nat. Mater. 2014, 13, 796; and Wu, R.; Yang, Y.; Li, M.; Qin, D.; Zhang,Y.; Hou, L. Solvent Engineering for High-Performance PbS Quantum DotsSolar Cells. Nanomaterials 2017, 7, 201). The research, however, hasmainly avoided or failed to address a number of parameters, such as thephotoactive layer thickness estimation, stability improvement, costreduction, and/or the layer deposition process. In optoelectronic devicefabrication, slot-die coating and screen printing has proven to be aproduct-compatible method for microfilm deposition. For thin-film (nanorange) photovoltaic devices, however, these methods are not applicable.A blade coating deposition method is generally used for thin filmdeposition, but QD solutions, such as a PbS QD solution, may not beviscous enough to implement a blade coating process (see, e.g., Zhang,J.; Gao, J.; Miller, E. M.; Luther, J. M.; Beard, M. C.Diffusion-Controlled Synthesis of PbS and PbSe Quantum Dots within SituHalide Passivation for Quantum Dot Solar Cells, ACS Nano 2014, 8,614-622).

For this reason and others, there are a number of challenges regardingQD photovoltaic (PV) device fabrication, including PbS QD PV devices.These challenges include, but are not limited to, [1] the active layerthickness for fabricating the device, [2] the ability to use layerdeposition methods other than spin coating, and/or [3] the high cost ofmost, if not all, QD solar cells, including PbS QD solar cells. Most, ifnot all, previous research suggests that PbS QDs at a high concentration(40 mg mL−1 to 100 mg mL−1) is required to fabricate a working device(see, e.g., Wang, H.; Kubo, T.; Nakazaki, J.; Kinoshita, T.; Segawa, H.PbS-Quantum-Dot-Based Heterojunction Solar Cells Utilizing ZnO Nanowiresfor High External Quantum Efficiency in the Near-Infrared Region, J.Phys. Chem. Lett. 2013, 4, 2455-2460, and Xu, W.; Tan, F.; Liu, Q.; Liu,X.; Jiang, Q.; Wei, L.; Zhang, W.; Wang, Z.; Qu, S.; Wang, Z. EfficientPbS QD solar cell with an inverted structure. Solar Energy Mater. SolarCells 2017, 159, 503 509). Although previous studies have used highlyconcentrated PbS QDs (e.g., 30-50 mg/mL), only low fill factors wereachieved (e.g., 40-50%) (see, e.g., Brown, P. R.; Lunt, R. R.; Zhao, N.;Osedach, T. P.; Wanger, D. D.; Chang, L.-Y.; Bawendi, M. G.; Bulovic',V. Improved Current Extraction from ZnO/PbS Quantum Dot HeterojunctionPhotovoltaics Using a MoO₃ Interfacial Layer, Nano Lett. 2011, 11,2955-2961). The relatively high concentration of QDs can increase thecost of the devices significantly, especially when a spin coatingprocess is used, because a relatively large amount of material is wastedduring spin coating. Another disadvantage of current QD solar cellsincludes the use of only an Ag material as a back electrode. Such a backelectrode does not provide device stability, because Ag may oxidizequickly to form an Ag₂O intermediate layer.

There remains a need for QD solar cells and methods of forming,optimizing, and/or simulating QD solar cells that address one or more ofthese disadvantages. For example, there remains a need for a low-cost,efficient, and/or more stable QD solar cell.

BRIEF SUMMARY

Provided herein are photovoltaic devices that address one or more of theforegoing needs, including photovoltaic devices that include PbS Qdsand/or a Cr—Ag electrode, which can provide an electrode surface withair stability. The photovoltaic devices provided herein may be low-costand/or stable. Methods of fabricating photovoltaic devices, includinglow-cost and/or stable photovoltaic devices, also are provided. Themethods may include one of two types of deposition method. The methodsprovided herein may be performed at ambient temperature and/or pressure.The methods provided herein may reduce the costs of producingphotovoltaic devices, at least in part, because the methods may depositliquids that include a relatively low concentration of QDs. The methodsalso may produce photovoltaic devices having one or more layers that areuniform, crack-free, and/or pinhole-free.

In one aspect, photovoltaic devices are provided. In some embodiments,the photovoltaic devices include a first electrode that includes (i) alayer including Cr, and (ii) at least one contact that (a) includes Ag,and (b) is arranged on the layer including Cr; an active layer having athickness of at least 350 nm, wherein the active layer includes (i) atleast one first quantum dot layer including PbS quantum dots treatedwith 1,2-ethanedithiol, wherein the layer including Cr is arrangedbetween the at least one contact and the at least one first quantum dotlayer, and (ii) at least one second quantum dot layer including PbSquantum dots treated with tetrabutylammonium iodide; at least onesemiconductor layer, wherein the at least one second quantum dot layeris arranged between the at least one first quantum dot layer and the atleast one semiconductor layer; and a second electrode, wherein the atleast one semiconductor layer is arranged between the at least onesecond quantum dot layer and the second electrode.

In some embodiments, the photovoltaic devices include a first electrode,a first layer stack, a second layer stack, wherein the first layer stackis arranged between the first electrode and the second layer stack, asecond electrode, wherein the second layer stack is arranged between thefirst layer stack and the second electrode, optionally a third layerstack arranged between the second layer stack and the second electrode,and optionally a fourth layer stack arranged between the third layerstack and the second electrode. The first, second, third, and fourthlayer stack may include a first, second, third, and fourth holetransporting layer, respectively, a first, second, third, and fourthactive layer, respectively, and a first, second, third, and fourthelectron transporting layer, respectively. The first, second, third, andfourth active layers may include one or more active materials. The oneor more active materials may be independently selected from the groupconsisting of poly(3-hexylthiophene-2,5-diyl) (P3HT), indene-C₆₀bisadduct (ICBA),poly([2,6′-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3-b]dithiophene]{3-fluoro-2[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl})(PTB7-Th), [6,6]-phenyl-C71-butyric acid methyl ester (PCMB),poly[2,7-(5,5-bis-(3,7-dimethyloctyl)-5H-dithieno[3,2-b:2′,3′-d]pyran)-alt-4,7-(5,6-dirluoro-2,1,3-benzothiadiazole) (PDTP-DFBT),poly[[2,5-bis(2-hexyldecyl-2,3,5,6-tetrahydro-3,6-dioxopyrrolo[3,4-c]pyrrole-1,4-diyl]-alt-[3′,3″-dimethyl-2,2′:5′,2″-terthiphene]-5,5″-diyl](PMDPP3T),poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl]](Si-PCPDTBT), methylammonium lead iodide (MaPbI₃), and lead (II) sulfide(PbS).

In another aspect, methods of fabricating photovoltaic devices areprovided. In some embodiments, the methods include performing one ormore optical simulations to select (i) a thickness of at least one ofthe first active layer, the second active layer, the third active layer,and the fourth active layer, and/or (ii) the one or more activematerials of at least one of the first active layer, the second activelayer, the third active layer, and the fourth active layer.

In some embodiments, the methods include (i) providing a semiconductorlayer arranged on a first electrode; (ii) disposing a first liquid and afirst amount of PbS quantum dots on the semiconductor layer; (iii)(a)drying the first liquid or (b) spin coating the semiconductor layer toremove substantially all of the first liquid from the first amount ofPbS quantum dots; (iv) disposing a first amount of tetrabutylammoniumiodide on the first amount of PbS quantum dots; (v) contacting thetetrabutylammonium iodide with a rinsing liquid to remove at least aportion of the tetrabutylammonium iodide from the first amount of PbSquantum dots; (vi) optionally repeating steps (ii) to (v) one or moretimes; (vii) disposing a second liquid including a second amount of PbSquantum dots on the first amount of PbS quantum dots; (viii)(a) dryingthe second liquid or (b) spin coating the semiconductor layer to removesubstantially all of the second liquid from the second amount of PbSquantum dots; (ix) disposing a first amount of 1,2-ethanedithiol on thesecond amount of PbS quantum dots; (x) contacting the 1,2-ethanedithiolwith the rinsing liquid to remove at least a portion of the1,2-ethanedithiol from the second amount of PbS quantum dots; (xi)optionally repeating steps (viii) to (x) one or more times; (xii)disposing a second electrode on the second amount of PbS quantum dots,wherein the disposing of the second electrode comprises (a) disposing alayer comprising Cr on the second amount of PbS quantum dots, and (b)disposing at least one contact comprising Ag on the layer comprising Cr.

In some embodiments, the methods of fabricating a photovoltaic deviceinclude using optical simulations to select the materials ofconstruction and their thicknesses to build active layers of an organicpolymer photovoltaic cell or a hybrid photovoltaic cell.

Additional aspects will be set forth in part in the description whichfollows, and in part will be obvious from the description, or may belearned by practice of the aspects described herein. The advantagesdescribed herein may be realized and attained by means of the elementsand combinations particularly pointed out in the appended claims. It isto be understood that both the foregoing general description and thefollowing detailed description are exemplary and explanatory only andare not restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a schematic of a layer arrangement of an embodiment of aphotovoltaic device.

FIG. 2 depicts a schematic of a layer arrangement of an embodiment of aphotovoltaic device.

FIG. 3 depicts a schematic of a layer arrangement of an embodiment of aphotovoltaic device.

FIG. 4 depicts a plot of current density versus thickness forembodiments of type 1 multi-junction polymer solar cells.

FIG. 5 is a HOMO and LUMO band diagram for an embodiment of a type 1multi-junction polymer solar cell.

FIG. 6 is a schematic of a layer arrangement for an embodiment of aphotovoltaic device.

FIG. 7 depicts a plot of light intensity fraction versus wavelength foran embodiment of a photovoltaic device.

FIG. 8 depicts a plot of current density versus load voltage for anembodiment of a photovoltaic device.

FIG. 9 is a schematic of a layer arrangement for an embodiment of aphotovoltaic device.

FIG. 10 is a schematic of a layer arrangement for an embodiment of aphotovoltaic device.

FIG. 11 is a schematic of a layer arrangement for an embodiment of aphotovoltaic device.

FIG. 12 is a schematic of a layer arrangement for an embodiment of aphotovoltaic device.

FIG. 13 is a schematic of a layer arrangement for an embodiment of aphotovoltaic device.

FIG. 14A depicts a schematic diagram of an embodiment of a device.

FIG. 14B depicts an energy level diagram of an embodiment of a PbS QDphotovoltaic device.

FIG. 15 depicts a plot of current density versus PbS layer thickness foran embodiment of a device.

FIG. 16A depicts the J-V characteristics of embodiments of PbS QD solarcells with two or three photoactive layers prepared using a drop castmethod.

FIG. 16B depicts the J-V characteristics of embodiments of PbS QD solarcells with four, five, or six photoactive layers prepared using a dropcast method.

FIG. 17 depicts the J-V characteristics of an embodiment of aseven-layered device fabricated, at least in part, by spin coating.

FIG. 18 depicts a dark J-V graph in log scale for embodiments(four-layered) of drop-cast and spin-coated devices.

FIG. 19 depicts a plot of the external quantum efficiency (EQE) of anembodiment of a spin-coated device.

DETAILED DESCRIPTION

Provided herein are photovoltaic devices, which may be referred to assolar cells.

In some embodiments, the photovoltaic devices include a first electrodethat includes (i) a layer that includes Cr, and (ii) at least onecontact that (a) includes Ag, and (b) is arranged on the layer includingCr; an active layer having a thickness of at least 350 nm, wherein theactive layer includes (i) at least one first quantum dot layer includingPbS quantum dots treated with 1,2-ethanedithiol, wherein the layerincluding Cr is arranged between the at least one contact and the atleast one first quantum dot layer, and (ii) at least one second quantumdot layer including PbS quantum dots treated with tetrabutylammoniumiodide; at least one semiconductor layer, wherein the at least onesecond quantum dot layer is arranged between the at least one firstquantum dot layer and the at least one semiconductor layer; and a secondelectrode, wherein the at least one semiconductor layer is arrangedbetween the at least one second quantum dot layer and the secondelectrode. In some embodiments, the active layer is in contact with (i)the layer including Cr, and (ii) the at least one semiconductor layer.

As used herein, the phrase “active layer” generally refers to a layerthat is photovoltaic, or, in other words, capable of producing anelectric current upon exposure to light, e.g., sunlight.

As used herein, the phrase “arranged between” does not connote that anytwo layers are necessarily in contact with each other; therefore, alayer that is “arranged between” two other layers may be in contact with(i) one of the other layers, (ii) both of the other layers, or (iii)neither of the other layers.

In some embodiments, the photovoltaic devices have a structure accordingto FIG. 1 , which is a schematic of an embodiment of a layerarrangement. The device 100 of FIG. 1 includes a first electrode 110.The first electrode 110 includes a layer that includes chromium 112, andtwo contacts that include silver 111. The device 100 also includes anactive layer 120 that includes at least one quantum dot layer 121treated with a first liquid, and at least one quantum dot layer 122treated with a second liquid. In some embodiments, the at least onequantum layer layers (121, 122) independently include quantum dot layerstreated with 1,2 ethanedithiol, tetrabutylammonium iodide, or acombination thereof. The device 100 also includes a semiconductor layer130, which may be a charge transporting layer, such as a holetransporting layer or electron transporting layer. The device 100 alsoincludes a second electrode 140 that is arranged on substrate 150. Thesubstrate 150 may be a transparent and/or flexible substrate. As usedherein, the term “transparent” refers to materials having a totaltransmittance of at least 90%, or at least 95%, or at least 98%.

The photovoltaic devices generally may include any number of the firstand second quantum dot layers. In some embodiments, the active layerincludes 1 or 2 of the at least one first quantum dot layers. In someembodiments, the active layer includes 1 to 5 of the at least one secondquantum dot layers. In some embodiments, the photovoltaic devices have astructure according to the schematic of FIG. 2 . The device 200 of FIG.2 includes a first electrode 110. The first electrode 110 includes alayer that includes chromium 112, and two contacts that includes silver111. The device 200 also includes an active layer 120 that includes twoquantum dot layers (121 a, 121 b) treated with a first liquid, and fivequantum dot layer (122 a, 122 b, 122 c, 122 d, 122 e) treated with asecond liquid. The device 200 also includes a semiconductor layer 130,which may be a charge transporting layer, such as a hole transportinglayer or electron transporting layer. The device 200 also includes asecond electrode 140 that is arranged on substrate 150. The substrate150 may be a transparent and/or flexible substrate.

The active layer of the photovoltaic device generally may have anythickness. The thickness of the active layer may be determined by themethods described in the Examples. In some embodiments, the active layerof the photovoltaic devices has a thickness of about 300 nm to about1,500 nm, about 350 nm to about 1,500 nm, about 380 nm to about 1,500nm, about 380 nm to about 1,000 nm, about 380 nm to about 750 nm, about380 nm to about 500 nm, or about 380 nm to about 400 nm.

The layer including Cr that may be included in the first electrode ofthe photovoltaic devices generally may have any thickness. In someembodiments, the layer including Cr has a thickness of about 2 nm toabout 10 nm, about 2 nm to about 8 nm, about 3 nm to about 7 nm, orabout 5 nm.

In some embodiments, the photovoltaic devices have a fill factor ofabout 20% to about 60%, about 30% to about 50%, or about 40% to about50%. The fill factor may be determined by the methods described in theExamples (see, e.g., Eq. 5).

The at least one semiconductor layer of the photovoltaic devices mayinclude a charge transporting layer, a charge blocking layer, or acombination thereof. In some embodiments, the at least one semiconductorlayer is an electron transporting layer. In some embodiments, the atleast one semiconductor layer comprises ZnO.

In some embodiments, the photovoltaic devices herein may include two ormore layer stacks arranged between a first electrode and a secondelectrodes, wherein each of the two or more layer stacks include a holetransport layer, an electron transport layer, and an active layerarranged between the hole transport layer and electron transport layer.

In some embodiments, the photovoltaic devices include a first electrode;a first layer stack; a second layer stack, wherein the first layer stackis arranged between the first electrode and the second layer stack; asecond electrode, wherein the second layer stack is arranged between thefirst layer stack and the second electrode; optionally a third layerstack arranged between the second layer stack and the second electrode;and optionally a fourth layer stack arranged between the third layerstack and the second electrode; wherein the first layer stack comprisesa first hole transporting layer, a first active layer, and a firstelectron transporting layer, wherein the first active layer is arrangedbetween the first hole transporting layer and the first electrontransporting layer; wherein the second layer stack comprises a secondhole transporting layer, a second active layer, and a second electrontransporting layer, wherein the second active layer is arranged betweenthe second hole transporting layer and the second electron transportinglayer; wherein the third layer stack, when present, comprises a thirdhole transporting layer, a third active layer, and a third electrontransporting layer, wherein the third active layer is arranged betweenthe third hole transporting layer and the third electron transportinglayer; and wherein the fourth layer stack, when present, comprises afourth hole transporting layer, a fourth active layer, and a fourthelectron transporting layer, wherein the fourth active layer is arrangedbetween the fourth hole transporting layer and the fourth electrontransporting layer.

In some embodiments, the photovoltaic devices have a structure accordingto the schematic of FIG. 3 . The device 300 of FIG. 3 includes asubstrate 310, a first electrode 320, a first layer stack 330, a secondlayer stack 340, and a second electrode 350. The first layer stack 330includes a first hole transport layer 331, a first active layer 332, anda first electron transport layer 333. The second layer stack 340includes a second hole transport layer 341, a second active layer 342,and a second electron transport layer 343. An embodiment of aphotovoltaic device that includes three layer stacks is depicted at FIG.12 , and an embodiment of a photovoltaic device that includes four layerstacks is depicted at FIG. 13 .

Each of the first, second, third, and fourth layer stacks may be thesame or different. When any two of the first, second, third, and fourthlayer stacks are “different”, then at least one of the layers of eachlayer stack is different. For example, as depicted at FIG. 6 , aphotovoltaic device may include three layer stacks, each including adifferent active layer, but identical hole and electron transportinglayers. As a further example, as depicted at FIG. 11 , the photovoltaicdevice may include two layer stacks, each have the same holetransporting layers, but different active layers and different electrontransporting layers.

In some embodiments, the first active layer, the second active layer,the third active layer, and the fourth active layer include one or moreactive materials that are independently selected from the groupconsisting of poly(3-hexylthiophene-2,5-diyl) (P3HT), indene-C₆₀bisadduct (ICBA),poly([2,6′-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3-b]dithiophene]{3-fluoro-2[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl})(PTB7-Th), [6,6]-phenyl-C71-butyric acid methyl ester (PCMB),poly[2,7-(5,5-bis-(3,7-dimethyloctyl)-5H-dithieno[3,2-b:2′,3′-d]pyran)-alt-4,7-(5,6-dirluoro-2,1,3-benzothiadiazole) (PDTP-DFBT),poly[[2,5-bis(2-hexyldecyl-2,3,5,6-tetrahydro-3,6-dioxopyrrolo[3,4-c]pyrrole-1,4-diyl]-alt-[3′,3″-dimethyl-2,2′:5′,2″-terthiphene]-5,5″-diyl](PMDPP3T),poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl]](Si-PCPDTBT), methylammonium lead iodide (MaPbI₃), and lead (II) sulfide(PbS).

Hole Transporting Layers

Generally, the hole transporting layers of the devices herein mayinclude any known hole transporting material. In some embodiments, thehole transporting layers include at least one hole transporting materialand at least one matrix material. The hole transporting material mayinclude one or more of poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS), nickel oxide (NiO), or a combination thereof.

Electron Transporting Layers

Generally, the electron transporting layers of the devices herein mayinclude any known electron transporting material. In some embodiments,the electron transporting layers include at least one electrontransporting material and at least one matrix material.

In some embodiments, the electron transporting material includes a metaloxide. Non-limiting of metal oxides include SnO₂, TiO₂, ZnO,[6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM), or a combinationthereof. The electron transporting material, in some embodiments,includes zinc oxide, titanium (IV) oxide, or a combination thereof.

Hole Transporting Materials

Generally, any known hole transporting material may be used in the holetransporting layers of the devices provided herein. In some embodiments,the hole transporting material is an inorganic hole transportingmaterial, an organic hole transporting material, or a combinationthereof. In some embodiments, the hole transporting material is apolymeric hole transporting material. In some embodiments, the holetransporting material is a small organic molecule hole transportingmaterial.

In some embodiments, the hole transporting materials includeN²,N²,N²′,N²′,N⁷,N⁷,N⁷′,N⁷′-octakis(4-methoxyphenyl)-9,9′-spirobi[9H-fluorene]-2,2′,7,7′-tetramine(Spiro-OMeTAD), polytriarylamine (PTAA), fluorine-dithiophene (FDT),Cu-phthalocyanine (CuPc), copper (I) thiocyanate (CuSCN), poly3-hexylthiophene (P3HT), poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS),poly[[(2,4-dimethylphenyl)imino]-1,4-phenylene(9,9-dioctyl-9H-fluorene-2,7-diyl)-1,4-phenylene](PF8-TAA), poly(9,9-dioctylfluorene) (PFO), polyaniline (PANI),poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)(PCPDTBT),N-(6-amino-2,4-dioxo-1-propylpyrimidin-5-yl)-N-(2-methoxyethyl)-2-phenylbutanamide(PDPP3T), or a combination thereof.

Matrix Materials

Any matrix material in which a charge transporting material or othermaterial may be dispersed can be used in the devices provided herein. Insome embodiments, the matrix material is transparent, flexible, or acombination thereof.

In some embodiments, the matrix material includes polydimethylsiloxane(PDMS), poly(methyl methacrylate) (PMMA), polystyrene, polycarbonate,polyurethane (PU), polyvinylidene fluoride (PVDF), or a combinationthereof.

Any amount of a charge transporting material or other material may bedispersed in a matrix material non-uniformly or substantially uniformly.In some embodiments, a weight ratio of a charge transporting material tomatrix material is about 0.1:1 to about 1:0.1, about 0.3:1 to about1:0.3, about 0.5:1 to about 1:0.5, about 0.8:1 to about 1:0.8, about0.9:1 to about 1:0.9, or about 1:1.

Substrate

The substrates of the devices herein may include any material. In someembodiments, the substrate is transparent, flexible, or a combinationthereof.

In some embodiments, the substrate includes a glass, such as SiO₂, or anorganic material, such as polyethylene terephthalate (PET),polydimethylsiloxane (PDMS), hydroxypropylcellulose (HPC), or acombination thereof.

Electrodes

The electrodes of the devices herein may include any conductivematerial. In some embodiments, a first electrode and/or second electrodeincludes indium tin oxide (ITO), In₂O₃/Au/Ag, Au, Ag, Al carbon-basedmaterial, Cr, or a combination thereof. The carbon-based material mayinclude carbon nanotubes, carbon nanofibers, or a combination thereof.In some embodiments, the carbon-based material includes a buckypaper.

Methods

Also provided herein are methods of fabricating a photovoltaic devices,including those described herein. In some embodiments, the methodsinclude (i) providing a semiconductor layer arranged on a firstelectrode; (ii) disposing a first liquid and a first amount of PbSquantum dots on the semiconductor layer; (iii)(a) drying the firstliquid or (b) spin coating the semiconductor layer to removesubstantially all of the first liquid from the first amount of PbSquantum dots; (iv) disposing a first amount of tetrabutylammonium iodideon the first amount of PbS quantum dots; (v) contacting thetetrabutylammonium iodide with a first rinsing liquid to remove at leasta portion of the tetrabutylammonium iodide from the first amount of PbSquantum dots; (vi) optionally repeating steps (ii) to (v) one or moretimes; (vii) disposing a second liquid and a second amount of PbSquantum dots on the first amount of PbS quantum dots; (viii)(a) dryingthe second liquid or (b) spin coating the semiconductor layer to removesubstantially all of the second liquid from the second amount of PbSquantum dots; (ix) disposing a first amount of 1,2-ethanedithiol on thesecond amount of PbS quantum dots; (x) contacting the 1,2-ethanedithiolwith a second rinsing liquid to remove at least a portion of the1,2-ethanedithiol from the second amount of PbS quantum dots; (xi)optionally repeating steps (viii) to (x) one or more times; (xii)disposing a second electrode on the second amount of PbS quantum dots,wherein the disposing of the second electrode comprises (a) disposing alayer comprising Cr on the second amount of PbS quantum dots, and (b)disposing at least one contact comprising Ag on the layer comprising Cr.

Generally, steps (ii) to (v) may be performed any number of times. Thenumber of times may be selected to fabricate a layer having one or moredesired properties, such as thickness. In some embodiments, steps (ii)to (v) are repeated 0 to 4 times. In other words, steps (ii) to (v) maybe performed one time, two times, three times, four times, or fivetimes.

Generally, steps (viii) to (x) may be performed any number of times. Thenumber of times may be selected to fabricate a layer having one or moredesired properties, such as thickness. In some embodiments, steps (viii)to (x) are repeated once. In other words, steps (viii) to (x) areperformed twice.

PbS quantum dots may be disposed in the first liquid and/or the secondliquid prior to depositing the first liquid and/or the second liquid.The concentration of the PbS quantum dots in the first liquid and/or thesecond liquid may be the same or different, and may be selected from anyconcentration. The concentration of PbS quantum dots may be a relativelylow concentration (less than or equal to 40 mg/mL) in the first liquidand/or the second liquid. In some embodiments, a concentration of thePbS quantum dots in the first liquid and/or the second liquid is about 5mg/mL to about 25 mg/mL. In some embodiments, a concentration of the PbSquantum dots in the first liquid and/or the second liquid is about 5mg/mL to about 15 mg/mL.

The disposing of the layer that includes Cr and the at least one contactthat includes Ag may be achieved by any known technique. In someembodiments, the disposing of the layer including Cr and the at leastone contact including Ag includes thermally evaporating Cr and Ag,respectively.

The first and/or second rinsing liquid may be any liquid that is capableof rinsing, i.e., removing, at least a portion of the 1,2-ethanedithiolor tetrabutylammonium iodide. In some embodiments, the first and/orsecond rinsing liquid includes acetonitrile. The first and secondrinsing liquid may the same or different.

In some embodiments, the semiconductor layer includes zinc oxide, andthe first electrode includes indium tin oxide.

The methods provided herein also include performing one or more opticalsimulations to select one or more features of a photovoltaic device. Insome embodiments, the methods include performing one or more opticalsimulations to select (i) a thickness of at least one of the firstactive layer, the second active layer, the third active layer, and thefourth active layer, and/or (ii) the one or more active materials of atleast one of the first active layer, the second active layer, the thirdactive layer, and the fourth active layer.

All referenced publications are incorporated herein by reference intheir entirety. Furthermore, where a definition or use of a term in areference, which is incorporated by reference herein, is inconsistent orcontrary to the definition of that term provided herein, the definitionof that term provided herein applies and the definition of that term inthe reference does not apply.

While certain aspects of conventional technologies have been discussedto facilitate disclosure of various embodiments, the Applicant in no waydisclaim these technical aspects, and it is contemplated that thepresent disclosure may encompass one or more of the conventionaltechnical aspects discussed herein.

The present disclosure may address one or more of the problems anddeficiencies of known methods and processes. However, it is contemplatedthat various embodiments may prove useful in addressing other problemsand deficiencies in a number of technical areas. Therefore, the presentdisclosure should not necessarily be construed as limited to addressingany of the particular problems or deficiencies discussed herein.

In this specification, where a document, act or item of knowledge isreferred to or discussed, this reference or discussion is not anadmission that the document, act or item of knowledge or any combinationthereof was at the priority date, publicly available, known to thepublic, part of common general knowledge, or otherwise constitutes priorart under the applicable statutory provisions; or is known to berelevant to an attempt to solve any problem with which thisspecification is concerned.

In the descriptions provided herein, the terms “includes,” “is,”“containing,” “having,” and “comprises” are used in an open-endedfashion, and thus should be interpreted to mean “including, but notlimited to.” When, for example, methods or devices are claimed ordescribed in terms of “comprising” various steps or components, themethods and devices can also “consist essentially of” or “consist of”the various steps or components, unless stated otherwise.

The terms “a,” “an,” and “the” are intended to include pluralalternatives, e.g., at least one. For instance, the disclosure of “afirst electrode,” “an active layer”, and the like, is meant to encompassone, or mixtures or combinations of more than one first electrode,active layer, and the like, unless otherwise specified.

Various numerical ranges may be disclosed herein. When Applicantdiscloses or claims a range of any type, Applicant's intent is todisclose or claim individually each possible number that such a rangecould reasonably encompass, including end points of the range as well asany sub-ranges and combinations of sub-ranges encompassed therein,unless otherwise specified. Moreover, all numerical end points of rangesdisclosed herein are approximate. As a representative example, Applicantdiscloses, in some embodiments, a concentration of the PbS quantum dotsin the first liquid and/or the second liquid is about 5 mg/mL to about15 mg/mL. This range should be interpreted as encompassing a minimumconcentration of about 5 mg/mL, a maximum concentration of about 15mg/mL, and further encompasses “about” each of 6 mg/mL, 7 mg/mL, 8mg/mL, 9 mg/mL, 10 mg/mL, 11 mg/mL, 12 mg/mL, 13 mg/mL, and 14 mg/mL,including any ranges and sub-ranges between any of these values.

As used herein, the term “about” means plus or minus 10% of thenumerical value of the number with which it is being used.

EXAMPLES

The present invention is further illustrated by the following examples,which are not to be construed in any way as imposing limitations uponthe scope thereof. On the contrary, it is to be clearly understood thatresort may be had to various other aspects, embodiments, modifications,and equivalents thereof which, after reading the description herein, maysuggest themselves to one of ordinary skill in the art without departingfrom the spirit of the present invention or the scope of the appendedclaims. Thus, other aspects of this invention will be apparent to thoseskilled in the art from consideration of the specification and practiceof the invention disclosed herein.

Example 1—Modeling of Photovoltaic Devices

In this example, it was demonstrated that a multi-junction hybrid solarcell can absorb light beyond 2500 nm and cover the whole solar spectrumwith 20% efficiency. A tandem polymer also was created with 12%efficiency. The device structures of this example were arranged in sucha way that the high band gap material on the top of the devices andlower band gap materials on the bottom of the devices were able toabsorb the near-infrared spectrum of light. The tandem solar cellvoltage was increased likely due to the multiple junctions, and thecurrent also increased likely due to the fact that it covered thenear-infrared spectrum, thereby increasing efficiency.

Theoretical Considerations: The organic and inorganic materials used inthe simulations of this example are depicted at the following table. Toreduce charge recombination, two different materials called the electrontransport layer (ETL) and hole transport layer (HTL) were used, whichcollected the electron and hole, respectively, after charge separationin the interface.

Organic and Inorganic Materials Used in the Simulation of this Example

Symbol Name; Description SiO₂ Silicon dioxide, glass ITO Indium tinoxide; electrode that collects hole/anode PEDOT:PSS Poly polystyrenesulfonate; HTL P3HT Poly(3-hexylthiophene-2,5-diyl), electron donor ICBAIndene-C60 bisadduct, electron acceptor TiO₂ Titanium (IV) oxide, ETLPTB7-Th Poly([2,6′-4,8-di(5-ethyIhexylthienyl) benzo[1,2-b;3,3-b]dithiophene] [3-fluoro-2[(2-ethylhexy]) carbonyl] thieno(3,4-b]thiophenediyl]), electron donor PCBM [6,6]-phenyl-C71-butyric acidmethyl ester, electron acceptor PDTP-DFBTPoly[2,7-(5,5-bis-(3,7-dimethyloctyl)-5H-dithieno[3,2-b:2′,3′-d]pyran)-alt-4,7-(5,6-difluoro-2,1,3 benzothia diazole); electron donor AlAluminum; electrode that collects electron/cathode PMDPP3TPoly[[2,5-bis(2-hexyldecyl-2,3,5,6-tetrahydro-3,6-dioxopyrrolo[3,4-c]pyrrole-1,4-diyl]-alt-[3′,3″-dimethyl-2,2′;5′,2″-terthiophene]-5,5″-diyl]; electron donorSi-PCPDTBTPoly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′] dithiophene-2,6-diyl]]; electron donor MaPbI₃Methylammonium lead iodide; semiconducting organic-inorganic materialPbS Lead (II) sulphide; semiconducting inorganic material Zno Zincoxide; ETL Ag Silver; electrode that collects electron/cathode NiONickel (II) oxide; HTL

Solar Cell Modeling: Optical transfer matrix theory describes theoptical processes inside a thin film layer stack, which is used toevaluate the power conversion efficiency of multijunction photovoltaic.When light having the energy of a photon with angular frequency tostrikes on an organic solar cell, local energy dissipation take place.

The local energy dissipated in the organic solar cell at the point z isgiven by:

$\begin{matrix}{{Q(z)} = {\frac{1}{2}c\varepsilon_{0}{\alpha n}{❘{E(z)}❘}^{2}}} & {{Eq}.1}\end{matrix}$

where, ε₀ is the permittivity of vacuum, n is the real index ofrefraction, α is the absorption coefficient, k is the extinctioncoefficient, c is the speed of light, λ is the wavelength and E is theoptical electric field at the point z.

$\alpha = \frac{4\pi k}{\lambda}$

G (z,λ), the exciton generation rate as a function of depth andwavelength is given by:

${G\left( {z,\lambda} \right)} = \frac{Q\left( {z,\lambda} \right)}{\hslash\omega}$

where h is the plank constant and

$\hslash = {\frac{h}{2\pi}.}$

Finally, one obtains the exciton generation rate at a depth of z bysumming G (z, λ) over the visible spectrum:

$\begin{matrix}{{G(z)} = {\sum\limits_{\lambda = {300}}^{\lambda = {2500}}{G\left( {z,\lambda} \right)}}} & {{Eq}.2}\end{matrix}$

For active layer thickness=t, the current density (mA/cm²) J_(sc) underAM1.5 illumination assuming 100% internal quantum efficiency is givenby:

J _(sc) =q×∫ ₀ ^(t) G(z)dz  Eq.3

An equivalent bulk heterojunction solar cell circuit is described below.In this example, a serial resistance R_(s)=0.01Ω was considered, andrelated to contact and bulk semiconductor resistances and a shuntresistance of R_(sh)=1000Ω.

The J-V characteristics were obtained from following equation:

$\begin{matrix}{J = {J_{sc} - {J_{o}\left( {{\exp\left( \frac{q\left( {V + {Is}} \right)}{KT} \right)} - 1} \right)} - \frac{V + {1.R_{s}}}{R_{sh}}}} & {{Eq}.4}\end{matrix}$${{Here}{}J_{o}} = {J_{sc}/\left( {{\exp\left( \frac{q.V_{oc}}{KT} \right)} - 1} \right)}$

J_(o) is saturation current density (mA/cm²) under reverse bias, V_(oc):open circuit voltage, J_(sc): short circuit current density, q:elementary charge, k: Boltzmann constant, T: temperature (K), V: outputvoltage.

Another parameter is the fill factor FF defined as:

FF=P _(max)/(V _(oc) ·J _(sc))=(V _(max) ·J _(max))/(V _(oc) ·J_(sc))  Eq.5

A parameter regarding cell performances is the power conversionefficiency (PCE) η:

$\begin{matrix}{\eta = {\frac{V_{\max}.J_{\max}}{P_{in}} = {{FF}\frac{V_{oc}.J_{sc}}{P_{in}}}}} & {{Eq}.6}\end{matrix}$

wherein J_(max) and V_(max) are the current density and voltagecorresponding to the maximum power P_(max) delivered by the solar cell.Pin is the incident photon flux (in mWcm⁻²) corresponding to AM 1.5(i.e. Pin is 100 mWcm⁻²).

The organic absorber films can be regarded as a semiconductor likematerial, where the band gap corresponds to the difference between theLUMO (Lowest Unoccupied Molecular Orbital) and the HOMO (HighestOccupied Molecular Orbital). The Voc can be calculated by followingequation:

$\begin{matrix}{{V{oc}} = {\left\lbrack \frac{{HOM{O(D)}} - {LUM{O(A)}}}{q} \right\rbrack - {0.3V}}} & {{Eq}.7}\end{matrix}$

Here, HOMO (D) is highest molecular orbital of donor material and LUMO(A) is the lower molecular orbital of acceptor materials.

The blending of donor and acceptor material provided a bulkheterojunction active layer. The HOMO and LUMO levels of donors andacceptors used in this example are shown at the following table.

HOMO and LUMO level of donor and acceptor of materials MATERIAL LUMO(eV) HOMO (eV) PTB7-Th(donor) −3.61 −5.25 PCBM (acceptor) −3.9 −5.9PMDPP3T(donor) −3.6 −5.2 P3HT (donor) −3.1 −5 PCPDTBT (donor) −3.55 −5.3MAPbI₃ −3.93 −5.46 ICBA (acceptor) −3.74 −5.6 Si-PCPDTBT (donor) −3.55−5.3 PDTP-DFBT (donor) −3.64 −5.26

Results for Multi-Junction Polymer Solar Cells: Three types ofmulti-junction polymer solar cells were investigated. Based on differentactive layers, the cell was stacked. The J-V characteristics andvariation of light intensity on the cell were analyzed. The powerconversion efficiency was calculated from the J-V curve. These threetypes of multi-junction polymer solar cell were simulated.

Type 1: Multi-Junction Polymer Solar Cell. By using the high, medium,and low bandgap organic materials, the stack was arranged for solarcells. The number of active layers determined the number of junctions.For the first multi-junction polymer solar cell, three active layerswere used, and hence it was called the three-junction polymer solarcell.

During the first step, the optical properties were determine for a cellwhose dimensions were—Glass/ITO (110 nm)/PEDOT:PSS (25 nm)/P3HT:ICBA(190 nm)/TiO₂ (25 nm)/PEDOT:PSS (25 nm)/PTB7-Th:PCBM 270 nm)/TiO₂ (25nm)/PEDOT:PSS (25 nm)/PDTP-DFBT:PCBM (640 nm)/TiO₂ (25 nm)/Al (200 nm).The thickness of the active layer affected the open circuit voltage(V_(oc)) as well as the short circuit current (J_(sc)) and thus theoverall power conversion efficiency (PCE). To optimize various activelayers, a general rule of thumb was used: decreasing the active layerthickness would increase the V_(oc) due to shorter diffusion length,while increasing the thickness would increase the J_(sc). Thus, the cellwas optimized to obtain the maximum performance.

The active layer thickness was estimated by using the MATLAB™ code(Burkhard, G. F.; Hoke, E. T. Transfer Matrix Optical Modeling. McGeheeGroup (Stanford Univ). 2011)(“Stanford model”).

According to FIG. 4 , the maximum total current that was possible fromthe type 1 solar cell was 30 mA/cm². As the three junctions wereconnected in series, each junction could provide 10 mA/cm² to obtain amaximum current of 30 mA/cm² from the cell. FIG. 4 demonstrates that ifcertain thicknesses were set (190 nm for P3HT:ICBA active layer, 270 nmfor PTB7-Th:PCBM active layer and 640 nm for PDTP-DFBT:PCBM activelayer), it was possible to obtain 10 mA/cm² for each junction. Using theMATLAB™ code, all the multi-junction polymer and hybrid cells optimizedthicknesses were calculated. The HOMO and LUMO band diagram for the type1 multi-junction PSC is depicted at FIG. 5 . As showed by FIG. 5 , withan active layer containing three different bandgaps, the open circuitvoltage V_(oc) was 2.368 V.

A stack diagram for the three-junction OSC is depicted at FIG. 6 , whichincludes three active layers of P3HT:ICBA (high bandgap), PTB7-Th:PCBM(medium bandgap) and PDTP-DFBT:PCBM (low bandgap) to cover the solarspectrum with wavelengths of 300-1000 nm, and the absorbed lightintensity of this device was above 70%. From the simulation of thisexample, it was determined that J_(sc) at each junction was 10.0194mA/cm². The J-V characteristics are shown at FIG. 8 . A PCE of 12.73%was achieved with this configuration.

Type 2: Multi-Junction Polymer Solar Cell. The optical properties of thesecond three-junction OSC were investigated. The dimensions of the cellare described as follows: Glass/ITO (110 nm)/PEDOT:PSS (25 nm)/P3HT:ICBA(235 nm)/TiO₂ (25 nm)/PEDOT:PSS (25 nm)/Si-PCPDTBT:PCBM (290 nm)/TiO₂(25 nm)/PEDOT:PSS (25 nm)/PMDPP3T:PCBM (1000 nm)/TiO₂ (25 nm)/Al (200nm). A HOMO and LUMO band diagram was produced, and it was determinedthat with an active layer of three different bandgaps, the open circuitvoltage V_(oc) was 2.07 V.

The stack diagram for the three-junction OSC is depicted at FIG. 9 .From a plot of light intensity fraction v. wavelength, it was noted thatthe three active layers of P3HT:ICBA, Si-PCPDTBT:PCBM and PMDPP3T:PCBM(low bandgap) covered the solar spectrum with wavelengths of 300-1000 nmand the light intensity was above 70%. J-V characteristics were alsoplotted. From the simulation, it was determined that J_(sc) at eachjunction was 10.1962 mA/cm². A PCE of 10.03% was achieved with thisconfiguration with an FF of 47.52%.

Type 3: Multi junction Polymer Solar Cell. The third three-junction OSCwas investigated. The dimensions of the cell were Glass/ITO (110nm)/PEDOT:PSS (25 nm)/P3HT:ICBA (200 nm)/TiO₂ (25 nm)/PEDOT:PSS (25nm)/Si-PCPDTBT:PCBM (290 nm)/TiO₂ (25 nm)/PEDOT:PSS (25nm)/PDTP-DFBT:PCBM (1000 nm)/TiO₂ (25 nm)/Al (200 nm). A HOMO and LUMOband diagram for the type 1 multi-junction PSC was produced. Based onthe active layer of three different bandgaps, it was determined that theopen circuit voltage Voc was 2.17 V.

The stack diagram for the three-junction OSC is shown at FIG. 10 . Froma plot of light intensity fraction v. wavelength, it was determined thatthe three active layers of P3HT:ICBA, Si-PCPDTBT:PCBM and PDTP-DFBT:PCBMcovered the solar spectrum with a wavelength in the range of 300-1000 nmand the light intensity was above 80%. J-V characteristics were alsoplotted. From the simulation, it was determined that the J_(sc) at eachjunction was 10.0130 mA/cm². A PCE of 10.90% was achieved using thisconfiguration with a FF of 50.17%.

Results for Two-, Three-, and Four-Junction Hybrid Solar Cells. Two-,three- and four-junction hybrid solar cells were investigated. Based ondifferent active layers, the cell was stacked. The J-V characteristicsand variation of light intensity on the cell were analyzed. The powerconversion efficiency was calculated from the J-V curve.

Two-Junction Hybrid Solar Cell: First, the two-junction hybrid solarcell (HSC) was simulated. The lead sulfide (PbS) was used as a lowbandgap material to absorb light beyond the infrared spectrum. Theactive layer MaPbI₃ was organic and inorganic, while PbS in HSC isinorganic in nature. During the first step, the optical properties weredetermined for a cell whose dimensions were: Glass/ITO (100nm)/PEDOT:PSS (20 nm)/P3HT:ICBA (300 nm)/TiO₂ (25 nm)/PEDOT:PSS (20nm)/MaPbI₃ (1110 nm)/TiO₂ (25 nm)/PEDOT:PSS (20 nm)/PbS (3000 nm)/ZnO(25 nm)/Ag (200 nm). The stack diagram for the three-junction OSC isdepicted at FIG. 11 . A plot of light intensity fraction v. wavelengthrevealed that the two active layers of MaPbI₃ and rear PbS active layerscovered the solar spectrum with wavelengths of 300 nm-2500 nm and thelight intensity was above 80%. A plot of generation rate v. position indevice revealed that only the active layers created excitons.

Three-Junction Hybrid Solar Cell: The second solar cell investigated wasa three-junction HSC. The active layer of P3HT:ICBA was organic butMaPbI₃ was both inorganic and organic, and PbS was inorganic in nature.During the first step, the optical properties were determined of a cellwhose dimensions were given by: Glass/ITO (100 nm)/PEDOT:PSS (30nm)/P3HT:ICBA (2000 nm)/TiO₂(15 nm)/NiO (20 nm)/MAPbI₃(1700 nm)/TiO₂(15nm)/NiO (20 nm)/PbS (1000 nm)/ZnO (15 nm)/Ag (200 nm). The stack diagramfor the three-junction OSC is depicted at FIG. 12 . From a plot of lightintensity fraction v. wavelength, it was determined that the threeactive layers of P3HT:ICBA, MaPbI₃ and rear PbS active layers coveredthe solar spectrum with wavelengths of 300 nm-2500 nm and the lightintensity was above 80%. A plot of generation rate v. position in devicerevealed that the three active layers were producing excitons.

Four-Junction Hybrid Solar Cell: The third solar cell that wasinvestigated was a four-junction HSC. During the first step, the opticalproperties were determined for a cell whose dimensions were as follows:Glass/ITO (100 nm)/PEDOT:PSS (20 nm)/P3HT:ICBA (500 nm)/TiO₂ (15 nm)/NiO(20 nm)/PTB7-Th:PCBM (2000 nm)/TiO₂ (20 nm)/NiO (15 nm)/PMDPP3T:PCBM(1100 nm)/TiO₂ (20 nm)/NiO (15 nm)/PbS (1000 nm)/ZnO (20 nm)/Ag (200nm). The stack diagram for the three-junction OSC is depicted at FIG. 13. From a plot of light intensity fraction v. wavelength, it wasdetermined that the three active layers of P3HT:ICBA, PTB7-Th:PCBM,PMDPP3T:PCBM and PbS covered the solar spectrum with wavelengths of 300nm-2500 nm and the light intensity was above 80%. From a plot ofgeneration rate v. position in device, it was observed that the fouractive layers are producing excitons.

Result analysis for three types of multi junction PSC and HSC: For thethree types of multi-junction polymer solar cells, the efficiency v.fill factor were plotted. It was observed that the efficiency for allthree types of PSCs were above 10%. However, the P3HT:ICBA, PTB7-Th:PCBMand PDTP-DFBT:PCBM three-junction PSC had a 12% efficiency. The J-Vcharacteristics for two-, three- and four-junction hybrid solar cellsalso were plotted, and the plots indicated that two junctions produce ahigh J_(sc) of 30 mA/cm² and four junctions produced a high V_(oc) of2.8 V. Another plot revealed that the power conversion efficiency wasabove 20% for the two-, three- and four-junction hybrid solar cells. Itwas observed that MAPbI₃ and PbS two-junction hybrid solar cell provideda 22% efficiency with a 55% fill factor.

As the fill factor was close to 50% for all cells, there was reasonableseries resistance (Rs) and parallel resistance (Rsh) during thecalculation of current density.

Theoretical settings were shown to improve the efficiency of the organicsolar cells, which were determined by optical modeling using thetransfer matrix. In multi-junction cells, the junctions were connectedin series, hence each junction current was equal. By varying the activelayer, the optimum current was found for each junction. The open circuitvoltage was calculated by the HOMO and LUMO levels of the materials,which were used as active layers in OSCs and HSCs. The simulations wereperformed using the Stanford model. The performance of the devicesvaried with the active layer thickness. From the results of thesimulations of this example, it was demonstrated that the multi-junctionpolymer solar cell and hybrid polymer solar cell could provide highefficiencies. The maximum PCE of 12.73% was achieved from themulti-junction polymer solar cell with the three active layers ofP3HT:ICBA , Si-PCPDTBT:PCBM and PMDPP3T:PCBM. Lead sulfide (PbS) wasshown to be the most promising low bandgap inorganic material, as it canabsorb sunlight beyond the infrared spectrum. By using inorganic PbS andhigh band organic materials, a solar cell that absorbs sunlight beyondwavelengths of 2500 nm was devised (see Example 2). The two-, three- andfour-junction hybrid solar cells provided a PCE above 20%. Thetwo-junction hybrid solar cell provided a high current of 30 mA/cm² andthe four-junction hybrid solar cell provided a high voltage of 2.8 V.

Example 2 Fabrication of PbS QD Photovoltaic Devices

Prior to the fabrication of the device, the MATLAB™ simulation ofExample 1 was created and performed. The optimized thickness that wouldincrease or maximum current density was estimated. Then, the device wasfabricated, and the device was an inverted-structure ZnO/PbS QD devicethat aimed to attain the simulated optimized active layer (PbS QD)thickness. Several devices were fabricated and compared to analyze theperformance of PbS QD solar cells using drop cast and spin coatingmethodologies. For cost optimization, 10 mg mL⁻¹ PbS QDs were usedinstead of highly concentrated PbS QDs. In an attempt to improve the airstability performance of the back electrode, a Cr—Ag electrode was usedbecause it was believed that the Cr layer would provide stability andsticks to the surface of the cell. Thin layers of Cr (5-10 nm) did notusually modify the properties of the devices.

Materials: ITO (Indium Tin Oxide) coated glass substrates (110 nm, 8-12Ω/sq), PbS core-type quantum dots, 1,2-ethanedithiol (EDT),tetrabutylammonium iodide (TBAI), and all liquids were obtained fromSigma-Aldrich (St. Louis, MO, USA), and were used without additionalrefinement or alteration. Cr (99.9%) and Ag (99.9%) pellets wereobtained from Lesker (Jefferson Hills, PA, USA).

Estimation of the PbS QD Layer Thickness: The structure of the PbS QDsolar cells of this example was ITO/ZnO/PbS-TBAI/PbS-EDT/Cr/Ag. Aschematic and band diagram for a device of this example is depicted atFIG. 14A and FIG. 14B, respectively.

The thickness of the PbS QD layer was estimated using the processreported by Khanam, J. J.; Foo, S. Y. Modeling of High-EfficjencyMulti-Junction Polymer and Hybrid Solar Cells to Absorb Infrared Light,Polymers 2019, 11, 383. The MATLAB™ simulation processed the same devicestructure shown at FIG. 14A. By varying the thickness of the PbS, it wasfound, in this example, that a PbS thickness of 380 nm provided themaximum current density of 43 mA/cm² (FIG. 15 ). The current density wasobtained from 100% Internal Quantum Efficiency (IQE).

Device Fabrication: On the ITO coated glass substrates, devices havingthe following structure were fabricated:(ITO/ZnO/PbS-TBAI/PbS-EDT/Cr/Ag). 1 M HCl was used in the etchingprocess of the ITO coated glass in order to avoid, or at least limit,short circuiting. A detergent was first used to clean the substrate.Deionized water, isopropanol, and acetone were subsequently used toclean the substrate. The substrates were dried in a vacuum oven andsubjected to an oxygen plasma treatment for about 5 minutes.

A ZnO nanoparticle solution was spin coated at 2000 rpm on the substratefor approximately 20 seconds to produce an 80 nm thick ZnO layer. TheITO/ZnO substrate was then heated for 20 minutes at 110° C.

For both spin coating and drop cast device fabrication,oleic-acid-coated PbS QDs with a concentration of 10 mg/mL in toluenesolvent were used. For the ligand exchange process, 1,2-ethanedithiol(EDT) and tetrabutylammonium iodide (TBAI) were used. As organic andinorganic ligands, an EDT solution (0.04 vol % in acetonitrile (ACN))and a TBAI solution (10 mg mL⁻¹ in methanol) were used, respectively.Due to ligand interchange, the device lost some surface layer volume.This generated cracks on the surface. To remove those cracks, ACN wasused as a rinsing solvent. The spin coating and drop cast layerdeposition methods were performed in open air and at ambienttemperature.

Drop Cast Deposition Method for Device Fabrication: Active layers weredeposited on the ITO/ZnO substrate using layer-by-layer (LbL) depositionof the drop cast method. Two-, three-, four-, five-, and six-layereddevices were fabricated using the drop cast method to determine whichlayer number increased and/or maximized efficiency. For the PbS-TBAIphotoactive layer deposition, about 30 μL of PbS QDs were dropped andallowed to dry completely on the ITO/ZnO substrate. Then, a TBAIsolution was dropped onto the substrate and left for 40 seconds. Thesubstrate was rinsed two times using ACN at 2000 rpm.

For the PbS-EDT photoactive layer deposition, about 30 μL of PbS QDswere dropped and allowed to dry completely on the ITO/ZnO substrate.After that, EDT solution was dropped onto the substrate and left forabout 40 seconds. The same rinsing process as above was used.

Finally, the devices were heated at 110° C. for 5 minutes. The layeredsubstrate was preserved in open air overnight. After that, for electrodeevaporation, the substrate was moved to a nitrogen (N₂)-filled glovebox. For thermal evaporation, the device was covered on the edges withKapton tape and mounted on a sample holder with carbon tape. The chamberwas pumped down to a 5×10⁻⁷ torr base pressure before evaporation. Then,5 nm Cr and 100 nm Ag were thermally evaporated at rates of 0.7 Å s⁻¹and 1 Å s⁻¹, respectively, at reduced pressure (<10⁻⁶ Torr). Duringthermal evaporation, a shadow mask was used. The photoactive area of thedevices of this example was about 1 mm².

The Spin Coating Deposition Method for Device Fabrication: In the spincoating method, the photoactive layers were fabricated on the ITO/ZnOsubstrate by LBL deposition. A seven-layered device was fabricated withthis method.

For the PbS-TBAI photoactive layers on the device, about 30 μL of PbSQDs were dropped onto the ITO/ZnO substrate for 90 seconds to adherewell to the glass. Then, the substrate was spin coated for 10 seconds at2500 rpm. Then, a TBAI solution was dropped onto the substrate and leftfor about 40 seconds. The substrate was subsequently rinsed two timesusing ACN at 2000 rpm. For the PbS-EDT photoactive layer deposition,about 30 μL of PbS QDs was dropped and left for 90 seconds on theITO/ZnO substrate. Then, the substrate was spin coated for 15 seconds at2500 rpm. After that, an EDT solution was dropped onto the substrate andleft for about 40 seconds. The same rinsing process as above was used.After deposition of two to three photoactive layers, the substrate washeated at 80° C. for 5 minutes. Then, the Cr—Ag electrode was depositedusing thermal evaporation. The active device area was about 3 mm².

Device Characterization and Instrumentation: The current density-voltage(J-V) characteristics of the devices were measured to determine theirefficiency. A KEITHLEY™ 2400 was used instrument at light intensity 100mW/cm² to measure the J-V characteristics.

The surface structure and a cross-sectional view using a field emissionscanning electron microscope (FESEM) were investigated. A UV-vis-NIRspectrophotometer (PerkinElmer Co., Waltham, MA, USA) was used to obtainthe absorption spectra. The optical images were captured using anOLYMPUS' BX40 microscope with a 5× lens and a CCD camera (TeledynePhotometrics Co., Tucson, AZ, USA).

The surface film morphologies of both the drop cast and spin-coated PbSQDs were investigated using FESEM with different magnifications. For thedrop cast deposition method, several cracks were observed on the filmsurface of the device. However, for the spin coating deposition method,the device surface was substantially crack free. In the spin-coateddevice, the film surface, although crack-free, was not uniform.

To assist with the achievement of a uniform surface, the devices wereannealed after active layer deposition. Additionally or alternatively,the PbS QD materials were made more adhesive using a viscous solvent inorder to improve the surface quality.

A cross-sectional view was collected to explore the devices and estimatethe thicknesses of the layers. Large cracks and non-uniformity wereobserved in the cross-sectional view of a drop-cast device. In aspin-coated device, the film thickness was 802 nm, and the cross sectionwas crack free.

The J-V characteristics of the photovoltaic devices for different layersmade using the drop cast method are depicted at FIG. 16A and FIG. 16B.

The following table indicates that the drop-cast devices consisting oftwo, three, four, five, and six layers of PbS showed power conversionefficiency (PCE) values of 0%, 0%, 1.5%, 0.55%, and 0.2%, respectively.

Device Parameters Obtained from Spin Coating and Drop Cast Methods.

Deposition Layer Voc Jsc Fill Factor PCE Process Deposition (V) (mA/cm²)(FF) (%) (%) Spin coating 5 PbS-TBAI + 2 0.38 35 50 6.5 PbS-EDT (7Layers) Drop cast 1 PbS-TBAI + 1 0 0 0 0 PbS-EDT (2 layers) 2 PbS-TBAI +1 0 0 0 0 PbS-EDT (3 layers) 2 PbS-TBAI + 2 0.4 7.5 50 1.5 PbS-EDT (4layers) 3 PbS-TBAI + 2 0.4 3 46 0.55 PbS-EDT (5 layers) 4 PbS-TBAI + 20.4 1 48 0.2 PbS-EDT (6 Layers)

It was also observed that the open-circuit voltage (V_(oc)) was 0.4 V,but the current density (J_(sc)) varied in the drop-cast working devices(with four, five, and six layers). The four-layered PbS device showed a1.5% PCE. The results showed that the drop cast method could provide aworking device, but with very low efficiency.

The J-V characteristics of the photovoltaic devices made using the spincoating method are depicted at FIG. 17 . The device that included sevenlayers of PbS exhibited a PCE of 6.5%. A previously reported PCE was6.0% for a device (FTO/TiO₂/PbS) that used an Au/Ag anode (Tang, J.;Kemp, K. W.; Hoogland, S.; Jeong, K. S.; Liu, H.; Levina, L.; Furukawa,M.; Wang, X.; Debnath, R.; Cha, D.; et al. Colloidal-quantum-dotphotovoltaics using atomic-ligand passivation, Nat. Mater. 2011, 10,765). Hence, a device of this example exhibited an improvement with theCr/Ag electrode. The foregoing table also demonstrates that aspin-coated device of this example achieved a V_(oc) of about 0.38 V,current density (J_(sc)) of about 35 mA/cm², and FF of about 0.5.

For both the spin-coated and the drop-cast devices of this example, lowFF values were observed (e.g., less than 50%).

In this example, an N₂-filled glove box with controlled relativehumidity was used during the fabrication of the device.

The dark J-V graphs in log scale for both the drop-cast (four-layered)and spin-coated working devices of this example are depicted at FIG. 18. This figure shows that the leakage current was much lower for both thespin-coated and drop-cast devices of this example. In the forwardingdirection, both devices showed a very small current density, whichrepresented a higher rectification ratio. Therefore, both devices hadexcellent dark J-V characteristics.

A stability test was conducted on the spin-coated device. The deviceshowed air exposure stability over five days without any encapsulation.The device was preserved in an N₂-filled glovebox, and during stabilitytesting, the device was exposed to open air. On the first day, the PCEbecame high; however, on the remaining days, the PCE was stable.

MATLAB™ simulation results on the external quantum efficiency (EQE) ofthe simulated device indicated that the device absorbed the lightspectrum from 400 nm to 1600 nm. The EQE of the fabricated device alsoshowed a similar outcome, as depicted at FIG. 19 .

To investigate the necessity of LbL spin coating deposition, opticalimages were collected of the one- and five-layer spin-coated filmsurfaces. The one-layer spin-coated PbS surface showed many pinholes,whereas the five-layer spin coating showed no pinholes on the surface.

Also analyzed was the stability of back electrodes containing a Cr—Aglayer and those of only Ag during exposure to air. Optical images of thedevices were collected after five days of air exposure. It was observedthat the surface without Cr became cracked, whereas the Cr—Ag electrodewas stable and uniform.

That which is claimed is:
 1. A method of fabricating a photovoltaiccell, the method comprising: performing one or more optical simulationsand selecting, based on the one or more optical simulations— (i) athickness of at least one of a first active layer, a second activelayer, optionally a third active layer, and optionally a fourth activelayer, (ii) one or more active materials of at least one of the firstactive layer, the second active layer, optionally the third activelayer, and optionally the fourth active layer, or (iii) a combinationthereof; wherein the photovoltaic cell comprises— a first electrode; afirst layer stack; a second layer stack, wherein the first layer stackis arranged between the first electrode and the second layer stack; asecond electrode, wherein the second layer stack is arranged between thefirst layer stack and the second electrode; optionally a third layerstack arranged between the second layer stack and the second electrode;and optionally a fourth layer stack arranged between the third layerstack and the second electrode; wherein the first layer stack comprisesa first hole transporting layer, the first active layer, and a firstelectron transporting layer, wherein the first active layer is arrangedbetween the first hole transporting layer and the first electrontransporting layer; wherein the second layer stack comprises a secondhole transporting layer, the second active layer, and a second electrontransporting layer, wherein the second active layer is arranged betweenthe second hole transporting layer and the second electron transportinglayer; wherein the third layer stack, when present, comprises a thirdhole transporting layer, the third active layer, and a third electrontransporting layer, wherein the third active layer is arranged betweenthe third hole transporting layer and the third electron transportinglayer; wherein the fourth layer stack, when present, comprises a fourthhole transporting layer, the fourth active layer, and a fourth electrontransporting layer, wherein the fourth active layer is arranged betweenthe fourth hole transporting layer and the fourth electron transportinglayer; and wherein the first active layer, the second active layer, thethird active layer, and the fourth active layer comprise one or moreactive materials that are independently selected from the groupconsisting of poly(3-hexylthiophene-2,5-diyl) (P3HT), indene-C₆₀bisadduct (ICBA),poly([2,6′-4,8-di(5-ethylhexylthienyl)benzo[1,2-b;3,3-b]dithiophene]{3-fluoro-2[(2-ethylhexyl)carbonyl]thieno[3,4-b] thiophenediyl})(PTB7-Th), [6,6]-phenyl-C71-butyric acid methyl ester (PCMB),poly[2,7-(5,5-bis-(3,7-dimethyloctyl)-5H-dithieno[3,2-b:2′,3′-d]pyran)-alt-4,7-(5,6-dirluoro-2,1,3-benzothiadiazole) (PDTP-DFBT),poly[[2,5-bis(2-hexyldecyl-2,3,5,6-tetrahydro-3,6-dioxopyrrolo[3,4-c]pyrrole-1,4-diyl]-alt-[3′,3″-dimethyl-2,2′:5′,2″-terthiphene]-5,5″-diyl](PMDPP3T),poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl]](Si-PCPDTBT), methylammonium lead iodide (MaPbI₃), and lead (II) sulfide(PbS).
 2. The method of claim 1, further comprising: providing the firstelectrode and the second electrode; and arranging between the firstelectrode and the second electrode the first layer stack, the secondlayer stack, optionally the third layer stack, and optionally the fourthlayer stack.
 3. The method of claim 1, wherein the first active layer,the second active layer, or the first active layer and the second activelayer comprises lead (II) sulfide (PbS).
 4. The method of claim 1,wherein the first active layer, the second active layer, or the firstactive layer and the second active layer comprisepoly(3-hexylthiophene-2,5-diyl) (P3HT), indene-C₆₀ bisadduct (ICBA),methylammonium lead iodide (MaPbI₃), lead (II) sulfide (PbS), or acombination thereof.
 5. The method of claim 1, wherein the first activelayer has a thickness of about 300 nm to about 1,500 nm.
 6. The methodof claim 1, wherein the first active layer has a thickness of about 380nm to about 500 nm.
 7. The method of claim 1, wherein the second activelayer has a thickness of about 300 nm to about 1,500 nm.
 8. The methodof claim 1, wherein the second active layer has a thickness of about 380nm to about 500 nm.
 9. The method of claim 1, wherein the first electrontransporting layer, the second electron transporting layer, or the firstelectron transporting layer and the second electron transporting layercomprise a metal oxide.
 10. The method of claim 9, wherein the metaloxide is selected from the group consisting of SnO₂, TiO₂, and ZnO. 11.The method of claim 1, wherein the first electron transporting layer,the second electron transporting layer, or the first electrontransporting layer and the second electron transporting layer comprise[6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM).
 12. The method ofclaim 1, wherein the first hole transporting layer, the second holetransporting layer, or the first hole transporting layer and the secondhole transporting layer comprises nickel oxide (NiO),N²,N²,N²′,N²′,N⁷,N⁷,N⁷′,N⁷′-octakis(4-methoxyphenyl)-9,9′-spirobi[9H-fluorene]-2,2′,7,7′-tetramine(Spiro-OMeTAD), polytriarylamine (PTAA), fluorine-dithiophene (FDT),Cu-phthalocyanine (CuPc), copper (I) thiocyanate (CuSCN), poly3-hexylthiophene (P3HT), poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS),poly[[(2,4-dimethylphenyl)imino]-1,4-phenylene(9,9-dioctyl-9H-fluorene-2,7-diyl)-1,4-phenylene](PF8-TAA),poly(9,9-dioctylfluorene) (PFO), polyaniline (PANI),poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-T,1′,3′-benzothiadiazole)(PCPDTBT),N-(6-amino-2,4-dioxo-1-propylpyrimidin-5-yl)-N-(2-methoxyethyl)-2-phenylbutanamide(PDPP3T), or a combination thereof.
 13. The method of claim 1, whereinthe photovoltaic device comprises the third layer stack arranged betweenthe second layer stack and the second electrode.
 14. The method of claim13, wherein the photovoltaic device comprises the fourth layer stackarranged between the third layer stack and the second electrode.
 15. Themethod of claim 14, wherein the first active layer, the second activelayer, the third active layer, the fourth active layer, or anycombination thereof comprises lead (II) sulfide (PbS).
 16. The method ofclaim 14, wherein the first active layer, the second active layer, thethird active layer, the fourth active layer, or any combination thereofcomprises poly(3-hexylthiophene-2,5-diyl) (P3HT), indene-C₆₀ bisadduct(ICBA), methylammonium lead iodide (MaPbI₃), lead (II) sulfide (PbS), ora combination thereof.
 17. The method of claim 14, wherein the firstactive layer, the second active layer, the third active layer, and thefourth active layer, independently, have a thickness of about 300 nm toabout 1,500 nm.
 18. The method of claim 14, wherein the first activelayer, the second active layer, the third active layer, and the fourthactive layer, independently, have a thickness of about 380 nm to about500 nm.